Flow measuring arrangements measuring the flow of a medium through a measuring tube on the basis of pressure- or pressure difference measurements are applied in a large number of industrial plants, as well as in the water- and waste water industries.
For this, there are essentially two different measuring principles applied. These are the differential pressure method and the vortex-method.
In the case of the differential pressure method, the effect discovered by Daniel Bernoulli is used, according to which a constriction in a pipeline effects, dependent on flow through the pipe, a pressure difference between static pressure reigning in the measuring tube, in the direction of flow of the medium, before the constriction and after the constriction. The constriction is formed, for example, by apertures, orifices, nozzles, Venturi nozzles or Venturi tubes inserted into the measuring tube, and increases the flow velocity of the medium in this area, with an accompanying increase in the dynamic pressure. The flow through the measuring tube is proportional to the square root of the resulting pressure difference, which is referred to as the differential pressure. The differential pressure is measured by a pressure difference measuring transducer, which is connected to the measuring tube via a first pressure line before the constriction, and via a second pressure line after the constriction.
In the case of the vortex method, a bluff body is inserted into the measuring tube. The medium flows around the bluff body. Vortices are shed from the bluff body into the region behind the bluff body with a frequency dependent on the dimensions of the bluff body and dependent on the flow. These vortices, also known as Karman vortices, cause pressure fluctuations in the flow behind the bluff body in the measuring tube. These pressure fluctuations are recorded by a pressure measuring transducer, which measures the pressure at a location in the measuring tube downstream from the bluff body. The frequency of the shedding of vortices is reflected in the frequency of the changes in the measured pressure as a function of time. The frequency is derived on the basis of the measured pressure, and, from this, flow is determined.
In order to assure secure and reliable measurement of flow over very long periods of use, it is desirable that the measuring equipment be subjected to functionality testing sporadically, regularly, or as needed. In such case, users desire an automatic functionality check, in which the measuring arrangement is able to check its functional ability automatically without significantly changing flow-through the measuring tube.
In US 2002/0029130 A1, a diagnostic method for flow measuring systems using the differential pressure method is described. In this case, the difference between the currently measured pressure differential and the sliding average of the measured pressure differential is determined continuously. The statistical quantities of these differences, e.g. their average value and their standard deviation, are determined, and, from these, deductions on the state of the pressure supply lines in the measuring tube for the pressure difference measuring device can be derived. In such case, these statistical variables are compared with historical reference data. If this comparison registers a deviation from the reference data exceeding a predetermined threshold value, then, for example, an obstruction of the pressure supply lines has been detected.
In parallel therewith, or alternatively thereto, a spectral energy density of these differences is determined. Deviations of the spectral energy densities from historical reference data permit deductions on the state of the constriction effecting the pressure difference in the pipeline.
In DE 10 2005 055 285 A1, a diagnostic method for a pressure measuring transducer is described, in which the pressure to be measured externally acts on an isolating diaphragm and is transmitted via a hydraulic path integrated in the measuring transducer to a pressure sensor that measures this pressure. There, pressure fluctuations are impressed on the hydraulic path for the diagnosis; these pressure fluctuations are reflected in the measurement signal of the pressure sensor. For example, abrasion, corrosion, or accretion formation on the isolating diaphragm can be recognized on the basis of the effects of the pressure fluctuations on the measurement signal.
In U.S. Pat. No. 7,255,012 B2, a flow measuring arrangement is described, which has, installed in the measuring tube, an iris-type diaphragm with an adjustable aperture. The diaphragm serves as a valve, via which the desired flow through the measuring tube is set. For this purpose, the differential pressure falling across the diaphragm is measured and, from that, flow is determined. The measured flow is used in a feedback loop in such a manner as to set the opening of the diaphragm so that the desired flow is present.
Moreover, a diagnostic method for this flow measuring arrangement is described, in the case of which the size of the aperture of the diaphragm is changed for an instant, and the change of the measured pressure difference associated therewith is measured.
Since the diaphragm is used as a valve here, the system is relatively slow-acting and is unsuitable, for example, for bringing about fast, periodic changes in the size of the aperture. Moreover, large changes to aperture size effect a marked change of the flow.